The present application relates generally to cryptographic systems and, more particularly, to methods for providing cryptographic key recovery in such systems.
With each passing day, more and more computers are connected together through pervasive open networks, such as the Internet, Wide Area Networks (WANs), and the like. With the ever-increasing popularity of such environments comes the need for exchanging messages and other documents in a secured fashion over an open communication network. To this end, some sort of cryptographic systems is usually employed.
Generally, cryptographic systems use either "secret-key" encryption or "public key" encryption. In "secret-key" encryption, a single key is used for both encryption and decryption. Consider, for example, a user (sender) who wants to send an e-mail message to a colleague (recipient) in a secured manner, such that no one who intercepts the message will be able to read it. If the sender employs a cryptographic "secret key" to encrypt the message, the recipient, in turn, must also use the same key to decipher or decrypt the message. As a result, the same key must be initially transmitted via secure channels so that both parties can know it before encrypted messages can be sent over insecure channels. This is typically inconvenient, however. A better approach is, therefore, sought.
Public key cryptography overcomes the problem by eliminating the need for a single "secret" key. As illustrated in FIG. 1A, each user of a public key cryptographic system has two mathematically-related keys, a "public key" and a secret or "private key." Operating in a complementary fashion, each key in the pair unlocks the code that the other key makes. Knowing the public key does not help deduce the corresponding private key, however. Accordingly, the public key can be published and widely disseminated across a communications network, such as the Internet, without in any way compromising the integrity of the private key. Anyone can use a recipient's public key to encrypt a message to that person, with the recipient, in turn, using his or her own corresponding private key to decrypt the message. One's private key, on the other hand, is kept secret, known only to user.
Keys are typically stored on "keyrings." Public keys, including a user's own as well as those of colleagues', are stored in a "public keyring" file. A user's private key is, in a similar fashion, stored in a "private keyring" file. Each key pair has a User ID (such as the owner's name and e-mail address) so that the user and the user's colleagues can identify the owners of keys. Each private key also has a passphrase, or verbose password, that protects it. No one but a message's intended recipient can decrypt the message, not even the person who originally encrypted the message, because no one else has access to the private key necessary for decrypting the encrypted message.
Since public key cryptography provides privacy without the need for the same kind of secure channels that conventional secret key encryption requires, it is commonly employed to send secured messages and other documents from one individual to another across a network or other communication channel, including the Internet. An example of its use in a commercial product today includes PGP.TM., available from Pretty Good Privacy, Inc. of Santa Clara, Calif.
Keys are also used to digitally sign a message or file and, in a complementary manner, to verify a digital signature. These "digital signatures" allow authentication of messages. When a user signs a message, a cryptographic program uses that user's own private key to create a digital signature that is unique to both the contents of the message and the user's private key. Any recipient can employ the user's public key to authenticate the signature. Since the signer, alone, possesses the private key that created that signature, authentication of a signature confirms that the message was actually sent by the signer, and that the message has not been subsequently altered by anyone else. Forgery of a signed message is computationally infeasible.
By way of summary, FIG. 1B illustrates the functions for which public and private keys are used when sending and receiving messages. When keys are used to secure files stored on a user's own computer or local network server, the user is both the "sender" (the person who saves the file) and the "recipient" (the person who opens the file).
Cryptographic systems, including ones implementing public key cryptography, are described in the technical, trade, and patent literature. For a general description, see e.g., Schneier, Bruce, Applied Cryptography, Second Edition, John Wiley & Sons, Inc., 1996. For a description focusing on the PGP.TM. implementation of public key cryptography, see e.g., Garfinkel, Simon, PGP: Pretty Good Privacy, O'Reilly & Associates, Inc., 1995. The disclosures of each of the foregoing are hereby incorporated by reference.
Despite the benefits of public key cryptographic products, a particular problem arises in their everyday use, however. Specifically, oftentimes there exists a need for an authorized party other than the sender or the recipient to have access to an encrypted message since the recipient of the message might not always be available. Consider, for instance, an employee of a company who receives an important document for the company when an encrypted e-mail message. Further, the company needs access to the document in order to submit a timely proposal but the employee is not available, for instance the employee has left the company or is away on vacation. As perhaps a more common scenario, the employee is available but he or she has "lost" his or her private key, which is needed to decrypt the encrypted e-mail message. This can occur, for example, if the employee forgets the passphrase that is required to access his or her own private key. All told, there exists a need for additional techniques to decrypt an encrypted message so that authorized parties other than the recipient can have access to the encrypted message.